shell: respawn desktop-shell if it dies
shell: respawn desktop-shell if it dies

If the desktop-shell client goes away for any reason, respawn it. To
avoid harmful looping, limit the respawning to 5 times within 30
seconds, and then give up.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,Fantu/compositor-spice,krezovic/weston,giucam/weston,mchalupa/weston,jonnylamb/weston,jonnylamb/weston,sir-murray/weston,Gnurou/weston,udoprog/weston,Tarnyko/weston-xdg_surface_present,sir-murray/weston,xorgy/weston,Gnurou/weston,eyolfson/weston,krezovic/weston,giucam/weston,eyolfson/weston,mchalupa/weston,xorgy/weston,udoprog/weston,kwm81/weston,Fantu/compositor-spice,kwm81/weston